I have the same problem (see my post here: Re-flashing S08PT60 Using OSBDM On TWR-S08DC-PT60 ). The PT/RN60 devices require a Power-On Reset with the BGND pin held low to enter active background. Likely the reason the board worked for you several times, is that there was a valid user program that was running properly in the PT60. If there is not a valid program in the Flash or if the software runs away, the only way for the on-board OSBDM to regain control of the MCU is to hold the BGND pin low while cycling power on the PT60. Unfortunately, this board has a design flaw when using it in standalone mode with the on-board OSBDM interface. If you study the schematic, you will find there is no way to cycle power on the PT60 without disconnecting the USB cable, which also removes power from the OSBDM interface and hence causes it to "forget" what it was doing (i.e. holding the BGND pin low).

As I stated in my original post, when I worked for NXP/Freescale, I never could get the board to work with the OSBDM interface and ended up using a USB Multilink interface and powered the board through J8, where I could cycle power to the MCU.

It is a real shame that this board has such a serious design flaw, I wanted to use it to teach a basic microcontrollers class.